Fork-lift vehicles and like industrial trucks are commonly provided with an upright structure or mast, tower or pylon carried by the front of the vehicle and, in turn, provided with a lifting fork designed to engage beneath the supporting surface of a power to enable lifting of a load carried thereby. Various systems have been provided for elevating the fork and these include rack-and-pinion, hydraulic, nut-and-spindle and similar arrangements.
However, difficulties have been encountered with conventional lifting devices because they must satisfy a number of prerequisites. For example, the lifting device must not obscure the field of view of the vehicle operator and, on the other hand, must be sufficiently stable as to withstand the load forces and vibration disturbances which arise as the vehicle carries a load from one location to another. Furthermore, considerable structural strength is a prerequisite since the center of gravity of the load is frequently located ahead of the center of gravity of the lifting rack so that considerable bending stress is applied at least when large loads are carried.
Various solutions to the problem have been proposed and some review of the state of the art in this regard is imperative.
There are, for example, lifting racks for fork-lift and like industrial vehicles which comprise a pair of profiled uprights, i.e. uprights of U-cross-section, T-section and double T-section or more complicated profiles. Such profiles have been throught to be necessary to resist the considerable bending stresses which must be absorbed by the rack. A carriage is vertically shiftable on these uprights and they may cooperate with vertically displaceable beams of similar or more complicated profile which slidably interfit with the stationary profiles. While these racks do not materially limit the field of view of the operator and thus are advantageous in this regard, they are expensive and complicated to manufacture, are heavy and thus require increased structure in the vehicle upon which the rack is mounted, and when increased in size to withstand particularly massive loads, must be made so wide as to interfere with the operator's view of the work area ahead of the vehicle. With such racks, moreover, vibration and relative lateral movement of the interfitting parts is observed during motion of the vehicle and, indeed, the vehicle has relatively poor lateral stability. The vibration, chattering and like deficiencies arise in part because the bending movement in the plans of the uprights may not be sufficiently resisted by the profile members.
Moreover, when a loaded vehicle negotiates a turn or curve, momentum or inertia of the load applies lateral stresses to the structure which results in an impact of the movable parts of the frame and the stationary parts thereof since the latter invariably cooperate with considerable play therebetween.
The difficulty encountered with such arrangements is one of continuing compromises since the width of the uprights should be minimal to provide the fullest possible view for the operator, but a reduced width also involves reduced lateral stiffness and resistance to bending with increased instability in the overall structure.
The problem is multiplied by the fact that the system must be designed to carry heavy loads with centers of gravity offset from the plane of the uprights at distances well above the top of the stationary portion of the rack and even during motion of the vehicle. Hence, any elasticity in the coupling between the interfitting vertical members or uprights or within these members or uprights or within these members themselves can only lead to bending-type oscillations when the stability of the uprights, whether movable or stationary, is reduced to afford greater visibility.
There is also known a lift mechanism for a fork-lift truck or the like which comprises a single central mast having two telescopically interfitting parts, the mechanism being of the type known as a "MONO-MAST" in which the lifting system is simplified although stability is reduced and a serious impediment to proper viewing of the working zone is imposed by the central arrangement of the mast.
A single mast arrangement, moreover, is sensitive to bending stresses both in the plane transverse to the direction of displacement of the vehicle and parallel to this direction, sustains flexure-type oscillations of the system and generally limits the handling characteristics of the vehicle.